Elucidating Jet Energy Loss in Heavy Ion Collisions

Over the last eight years, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) has been providing heavy ion collisions in the attempt to create the Quark-Gluon Plasma (QGP) within the laboratory. Some of the most striking data from the first few years at RHIC were results from the apparent interaction of hard scattered partons in the medium produced at RHIC. Single high-pT particles are suppressed relative to binary collision-scaled p + p rates[1]; di-jets, measured from pairs of high-pT hadrons, are suppressed relative to p + p and have a strongly modified angular correlation[2]; and heavy quarks, both charm and bottom, are suppressed as well[3]. These data have been interpreted as the loss of energy of a colored parton traversing a colored medium, analogous to energy loss in QED. However, many of the details underlying QCD energy loss are not well understood. On the one hand, energy loss models are able to reproduce the single particle suppression with very different assumptions on the rate of energy loss[4]. On the other, multiple high-pT particle correlations may suffer from a bias for the jet to lose little if any energy[5, 6]. This is observed in data where the measured jet properties are quite similar between p+ p and A+A, and they have only slightly more constraining power on models. To overcome such biases, it is necessary to move beyond multi-particle correlations and measure jets directly in heavy ion collisions. This note highlights the current status of jet reconstruction and energy loss measurements expected to be made with the ATLAS detector at the Large Hadron Collider (LHC). The large acceptance and nearly hermetic electromagnetic and hadronic calorimeters were designed for jet measurements and are uniquely suited to perform these measurements in heavy ion collisions. Such measurments will result in an increased understanding of how colored partons lose energy in a colored environment, a direct test of QCD.